1892.] on Ojjtical Projection. 535 



Mr. Wriglit does not appear — if I may say so — to have had much ex- 

 perience with the electric arc light as a radiant, and I found, at a very 

 early stage, that great difficulties had to be encountered when this 

 light was used, chiefly because the radiant approaches more nearly to 

 what theory requires. That which was easy with the lime light 

 became almost impossible with the arc lamp, and these difficulties had 

 to be conquered. 



Many scientific men are dissatisfied with the projection microscope, 

 on the ground that very high magnification does not give that resolu- 

 tion and that sharpness which is found in the usual methods of 

 observation. This want I fully admit. At the same time it is 

 scarcely right to condemn a particular method, because you try to 

 apply it to an unsuitable purpose. Hundreds of thousands of subjects 

 may be shown with the projection microscope with far greater profit 

 to the student than was possible in the old way. The very fact that 

 the professor can place his pointer upon any part of the picture on 

 the screen is invaluable to the students. I shall, therefore, attempt to 

 show you only a series of microscopical subjects suitable for projec- 

 tion, and shall not employ very high magnification. 



In regard to some substances very high powers may be used with 

 advantage, but much time would be lost in getting them into the 

 field and focussing them upon the screen. These, consequently, I 

 omit, so that a larger number of subjects may be illustrated. 



It is fair to state that most of the apparatus used to-night has 

 been constructed by Messrs. Newton, of Fleet Street, and the luminous 

 pointer by Messrs. Steward, of the Strand. The arc lamp is a 

 Brockie's projector. Messrs. Baker, Watson, and others, have also 

 come to my assistance. 



I will first show, on the screen, a picture of the lantern carrying 

 its various apparatus ; and then a few systems of lenses, which may 

 be employed for the projection microscope, as well as a diagram of 

 the microscope itself. 



Sub-stage condensers and objectives are, as a rule, made to suit the 

 table microscope. When projecting, by means of an objective alone, 

 in consequence of the screen distance being very great— or, in other 

 words, the microscope tube being exceedingly long as compared with 

 the table instrument — the objective has to be approached very close 

 to the slide ; in fact, with the higher powers, closer than the cover- 

 glass will allow. This close working distance renders necessary 

 special sub-stage condensers, and in many cases a special one is 

 required for every screen distance with each objective. This requisite 

 would seem to be a complete stumbling-block to microscope projec- 

 tion work. With the limelight the difficulty does not enter in the same 

 degree as with the arc light, and as we are now dealing with the 

 latter, further reference need not be made to the oxy-hydrogen light. 

 There are two ways of surmounting the difficulty ; one by the use of 

 plano-concave lenses, introduced in such a way as to be equivalent to 

 greatly lengthening the focus of the objective on the screen side, 



